This invention relates to a method and apparatus for forming corrugated paperboard and more particularly to a method and apparatus for forming corrugated paperboard at a high rate of speed and of high quality.
Systems currently in use for forming corrugated paper board employ corrugating rollers for corrugating a corrugating medium, generally paper drawn off of a roll, to form a corrugated layer. In order to facilitate corrugation and to ensure that corrugations are formed and retain their shape, the paper is generally conditioned by the application of one or more steam showers applied upstream of the corrugating rollers. The corrugating medium is passed through the corrugating rollers, which are generally heated to temperatures above 340.degree. F., adhesive is applied to the crests of the corrugated paper flutes and linerboard is applied to form a single faced corrugated medium. If desired, the single faced corrugated medium is then double faced in order to enhance the strength of the corrugated product. A second linerboard is drawn over a preheating roller and joined with the exposed crests of the flutes of the single faced corrugated medium to which adhesive has been applied. The double faced corrugated medium is the passed over a heated medium or hot plate section while subjected to pressure from a weight roller and belt assembly above the hot plate section.
One problem in a conventional corrugating system is the occurrence of malformed flutes in the corrugated medium. When the corrugating medium is passed through the corrugating rolls, the corrugations are formed as the medium is pressed in the nip of the corrugating rolls under the application of heat and pressure to form flutes. A low flute occurs when the flute does not fully form in the nip of the corrugating rolls. The results of uneven amplitudes of the flutes are areas of little or no contact between the linerboard applied to the crests of the corrugated medium in subsequent double facing operations, thus forming a flawed corrugated product.
Another problem occurs when the flutes are deformed during subsequent processing. The main processing areas at which deformation occurs are in the removal of the corrugated medium from the corrugating rolls and in the hot plate section of a double facing system. Deformation during removal of the corrugated medium from the corrugating rolls results primarily from the inability of the corrugated medium to properly release from the corrugating rolls after the application of heat and pressure in the nip of the rolls.
Deformation also frequently occurs in the hot plate section of a double facing operation. When the second linerboard is applied to the adhesive on the exposed crests of the flutes of the single faced corrugated medium the resulting double faced corrugated medium is then passed over a hot plate section with pressure applied from a belt and weight roller assembly above the hot plates in order to cure the adhesive and to dry the paperboard. A high coefficient of friction between the hot plate section and the face of the linerboard results in deformed corrugation flutes as the double faced corrugated medium is subjected to shear forces between the roll press and the hot plate section surfaces.
The paperboard available in today's market is frequently recycled paper and it is expected that more recycled material will be supplied in the future. This recycled paper retains some undesirable additives and also is not always treated with a finishing agent. The result is a paperboard which obtains a high coefficient of friction when passed over the hot plates, thereby aggravating the flute deformation problem.
A high shear force in the double faced corrugagted medium also produces a loss of strength in the adhesive bond between the second linerboard and the crests of the flutes of the corrugated medium. The adhesive bond for the second linerboard is not set when the double faced corrugated medium enters the hot plates and high shear forces can break the initial bond so that slippage occurs between the second linerboard and the corrugated medium.
The result of malformed or deformed flutes is a reduction in the strength of the corrugated product. In today's market, corrugated products are generally manufactured to conform to various industry and government specifications. The loss in product quality resulting from such malformed and deformed flutes results in rejected goods, customer complaints, and returned shipments.
One way in which both of the above problems have been solved according to the prior art is to run the corrugating system at a low rate of production. This low rate of production is, however, significantly below the speeds at which the corrugating machines are capable of operating and is basically an unsatisfactory method for solving the problem. Obviously, a higher rate of production results in a more cost competitive product.
In addition, the prior art has attempted to solve the flute malformation problem by the use of various lubricants applied to the corrugating medium or to the corrugating rolls. As shown in U.S. Pat. Nos. 1,796,542, 3,676,247, and 3,103,459, the lubricants chosen are generally of a hydrocarbon base such as paraffin, wax, or polyethylene. Such hydrocarbon base lubricants have been applied in liquid form or have been formed as a solid, the use of a wax in solid bar form often being applied to the paper stock or to the crests of the corrugating rolls. These lubricants reduce the coefficient of friction between the paperboard and the corrugating rolls and do indeed result in some improvement in flute formation and higher operating speed. However, the use of such hydrocarbon lubricants results in additional problems. Such lubricants have a tendency to be absorbed by the paperboard or linerboard with a resulting discoloration of the paperboard or linerboard. The hydrocarbon lubricants also tend to vaporize in an oily smoke under the operating temperatures of the corrugating rolls and the hot plates. This oil vapor can result in unacceptable concentrations of hydrocarbons in the shop atmosphere. U.S. Pat. No. 3,676,247 further discloses a lubricant containing stearins for use in an unheated or low heat corrugating system but the wax base provided for the lubricant would be unacceptable in a heated system for the reasons discussed above.
It will also be noted from the references that the lubrication is applied to either the corrugating medium itself or to the crests of the flutes of the corrugating rolls. The lubrication, when applied to the corrugating rolls, is thus only a topical application and is not evenly distributed in the flutes of the corrugating rolls.
It should be noted that paper and paperboard has been treated with various products such as metallic stearates to impart a finish or waterproofing to such paper products as typically shown in U.S. Pat. No. 2,029,390. Metallic stearates have also been used as release agents in metal casting and plastic molding operations. Such agents have not heretofore been applied in the paper corrugating industry to improve corrugating system performance.
The disadvantages of the prior art are overcome in the present invention, however, and improved methods and apparatus are provided for forming a double faced corrugated medium that increases production rates with improved flute formation and reduced equipment operating costs.